1. Field of the Invention
The present invention relates to acoustic wave filter devices used in, for example, bandpass filters of mobile communication systems. More particularly, the present invention relates to a longitudinally coupled resonator-acoustic wave filter device having a balanced-unbalanced conversion function.
2. Description of the Related Art
Duplexers used in radio-frequency circuits in mobile phones are required to reduce the insertion loss in pass bands and to increase the amount of attenuation in attenuation bands near the pass bands. In addition, reception filters of the duplexers are required to have the balanced-unbalanced conversion function because of a reduction in the number of parts in the duplexers.
In order to satisfy such requirements, longitudinally coupled resonator-surface acoustic wave filters having the balanced-unbalanced conversion function are used as the reception filters of the duplexers.
Japanese Unexamined Patent Application Publication No. 2008-118277 described below discloses a duplexer including such a surface acoustic wave filter device. FIG. 11 is a schematic plan view for describing the structure of electrodes in the duplexer described in Japanese Unexamined Patent Application Publication No. 2008-118277.
In a duplexer 601, the structure of electrode shown in FIG. 11 is formed on a piezoelectric substrate 602. A longitudinally coupled resonator-surface acoustic wave filter unit 611 and a series resonator 607 connected in series to the longitudinally coupled resonator-surface acoustic wave filter unit 611 are connected between an unbalanced terminal 603, which is an antenna terminal connected to an antenna, and first and second balanced terminals 604 and 605, which are reception terminals. The longitudinally coupled resonator-surface acoustic wave filter unit 611 and the series resonator 607 form a reception filter. The longitudinally coupled resonator-surface acoustic wave filter unit 611 is a two-stage surface acoustic wave filter in which three-interdigital electrode transducer (IDT)-type longitudinally coupled resonator-surface acoustic wave filters 612 and 613 are cascaded to each other. The series resonator 607 is composed of a one-port surface acoustic wave resonator.
In contrast, a transmission filter 614 is connected between the unbalanced terminal 603, which is an antenna terminal, and a transmission terminal 606. The transmission filter 614 is composed of a ladder-type surface acoustic wave filter. Specifically, multiple surface acoustic wave resonators are connected to each other so as to have a ladder circuit configuration.
The duplexer 601 is a Universal Mobile Telecommunications System (UMTS)-BAND 8 duplexer and is used in a mobile phone. In the UMTS-BAND 8, the transmission pass band is from 880 MHz to 915 MHz and the reception pass band is from 925 MHz to 960 MHz. Since the frequency interval between the transmission pass band and the reception pass band is 10 MHz, the transmission pass band is close to the reception pass band. Accordingly, the reception filter of the duplexer 601 is required to have a larger amount of attenuation in the transmission pass band and to be excellent in sharpness of the filter characteristics. For this reason, the reception filter of the duplexer 601 uses the longitudinally coupled resonator-surface acoustic wave filter unit 611 having the two-stage structure in which the two longitudinally coupled resonator-surface acoustic wave filters 612 and 613 are cascaded to each other in order to increase the amount of attenuation in an attenuation band at a lower side of the pass band, which is the transmission pass band.
In addition, the series resonator 607 is connected in series to the longitudinally coupled resonator-surface acoustic wave filter unit 611 in the reception filter of the duplexer 601 because it is required to have a larger amount of attenuation in the attenuation band near the pass band. The resonant frequency of the series resonator 607 is positioned within the pass band of the reception filter and the anti-resonant frequency of the series resonator 607 is positioned within an attenuation band at a higher side of the pass band of the reception filter. The presence of the series resonator 607 allows the amount of attenuation in the attenuation band at the higher side of the pass band to be increased. In addition, parallel resonators may be connected between the unbalanced terminal 603 and a ground potential and between either of the first and second balanced terminals 604 and 605 and the ground potential, that is, the parallel resonators may be connected in parallel to the longitudinally coupled resonator-surface acoustic wave filter unit 611. In this case, the resonant frequency of the parallel resonators is positioned within the attenuation band at the lower side of the pass band of the reception filter and the anti-resonant frequency of the parallel resonators is positioned within the pass band of the reception filter. The presence of the parallel resonators allows the amount of attenuation in the attenuation band at the lower side of the pass band to be increased.
In structures in which multiple longitudinally coupled resonator-acoustic wave filters are cascaded to each other in acoustic wave filter devices, such as surface acoustic wave filter devices and boundary acoustic wave filter devices, the amount of attenuation in the attenuation band near the pass band is increased. However, the structures have a problem in that the insertion loss within the pass band is increased.
In order to decrease the insertion loss within the pass band, one-stage acoustic wave filter devices should be used, instead of the multi-stage acoustic wave filter devices in which multiple longitudinally coupled resonator-acoustic wave filters are cascaded to each other. However, with the one-stage acoustic wave filter devices, it is difficult to ensure a larger amount of attenuation in the attenuation band near the pass band. Accordingly, it is difficult to ensure both of the reduction in the insertion loss within the pass band and the increase in the amount of attenuation in the attenuation band near the pass band.
In contrast, as described above, the series resonator or the parallel resonator can be connected to the longitudinally coupled resonator-surface acoustic wave filter to increase the amount of attenuation in the attenuation band near the pass band and to improve the sharpness of the filter characteristics. However, when an acoustic wave filter device including a longitudinally coupled resonator-acoustic wave filter having the balanced-unbalanced conversion function and first and second parallel resonators connected between the respective first and second balanced terminals and the ground potential is used as the reception filter of a duplexer, the amount of attenuation in the transmission pass band can be decreased and/or the isolation can be apt to degrade, compared with a two-stage acoustic wave filter device in which two longitudinally coupled resonator-surface acoustic wave filters are cascaded to each other.